IT202100006257A1 - WATER RESISTANT MEMS PUSHBUTTON DEVICE, PACKAGE HOUSING THE PUSHBUTTON DEVICE AND PUSHBUTTON DEVICE MANUFACTURING METHOD - Google Patents

Description

DESCRIPTION

of the patent for industrial invention entitled:

?WATER RESISTANT MEMS PUSHBUTTON DEVICE, PACKAGE HOUSING THE PUSHBUTTON DEVICE AND PUSHBUTTON DEVICE MANUFACTURING METHOD?

The present invention ? relating to a push button device, a case, or package, housing the push button device and a method of manufacturing the push button device. In particular, the button device ? made in MEMS technology (?MicroElectroMechanical Systems?) and ? waterproof. Specifically, reference is made below to a button, which can be operated by a user to generate an electrical signal used by an electronic device for its operation.

As ? As known, input devices, such as keys, buttons or switches for portable electronic devices, such as smartphones and smartwatches, are typically physical tactile elements which allow the user to provide signals to the portable electronic device. For example, pressing a button on a smartphone allows a user to wake up the screen when in standby.

Known input devices comprise strain sensors (?strain?), which exploit different physical principles to detect the command given by the user; for example, strain sensors of known type are piezoresistive sensors, which detect a user command through a resistance variation caused by the application of an external force (for example, pressing the button) on the input device itself.

Currently, ? more and more Desired that the input devices be impermeable to fluids, typically water, to prevent failure of the portable electronic device due to fluid infiltration or to allow the device to be used in water, such as during water sports .

For this purpose, currently waterproof entry devices are equipped with sealing elements, such as O-rings, integrated into the assembly of portable electronic devices, which help prevent the entry of water inside the portable electronic device.

An example of an input device including an o-ring ? disclosed in US patent US 2015/0092345 A1 .

Another example of a known type of input device? described in US patent US 2016/0225551 A1 , which describes a portable electronic device including a physical button as an input device. Here, the button comprises a cap, movable in an enclosure of the portable electronic device, a flex member, coupled to the cap, and a strain sensor, coupled to the flex member. In use, an external force (for example, due to the user?s finger pressing on the cap) deflects the flex element, generating a corresponding stress in the strain sensor, which generates an electrical signal and delivers it to a processing element . However, this solution does not guarantee complete waterproofing of the button and also does not ? completely satisfactory.

Indeed, in the input devices for portable devices ? desired that they present a predefined stroke and a? hermeticity? durable over time, as well as small size.

However, the growing trend towards miniaturization of portable electronic devices? often incompatible with current ?waterproof? of the input devices; for example, current O-rings have non-negligible dimensions, which can interfere with the miniaturization requirement. In order to reconcile airtightness? and small size, the currently known input devices for portable electronic devices are complex both to manufacture and to assemble.

In addition, the current sealing elements are subject to wear, for example due to repeated stresses of the input device, as well as? to aging, with reduction of the capacity? sealing of the sealing elements.

Furthermore, the input devices of the known type usually have high power consumptions, which can significantly reduce the life of the battery of the portable electronic device.

Purpose of the present invention? solve the drawbacks of the prior art.

According to the present invention, a push-button device, a casing housing the push-button device and a manufacturing method of the push-button device, as defined in the attached claims, are provided.

For a better understanding of the present invention, embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, in which:

figure 1 shows, in cross section, a MEMS button structure, according to one aspect of the present invention;

- figure 2 shows, in plan view, the MEMS button structure of figure 1;

figure 3 shows, in cross section, the MEMS button structure of figure 1, coupled to a body of an external device, according to an aspect of the present invention;

- figures 4-10 show steps of a manufacturing process of the MEMS structure of figures 1 and 2;

figure 11 schematically shows a package in which the MEMS button structure of figures 1 and 2 and an ASIC controller are housed;

- figure 12 shows in schematic form the MEMS button structure of figures 1 and 2 housed on a flexible board.

Figure 1 shows, in side sectional view in a Cartesian coordinate system of X, Y, Z axes, an embodiment of a MEMS structure 50 suitable for being used as a button. Figure 2 shows, in top view on the XY plane, the MEMS structure 50.

With reference to both figures 1 and 2, the MEMS structure 50 comprises: a fixed structure 1, including a support frame 2, here by way of example of a closed quadrangular shape; and a movable structure 6 arranged inside the (that is, completely surrounded by) the support frame 2. The movable structure 6 ? partly mechanically coupled to the support frame 2 by at least one support region 14 having the shape of a ?beam? or ?trampoline? (?cantilever?), which protrudes from a portion of the mobile structure 6.

The support frame 2 has a thickness, measured along the Z axis, of between 50 and 500 µm.

The mobile structure 6 includes: a pillar (or piston) 4, free to move, under the action of a force acting on it, along the Z axis (for this purpose, the pillar 4 is not directly constrained to the frame of support 2); a ring 8, by way of example quadrangular in shape, arranged inside the support frame 1 and in turn surrounding the pillar 4 (the ring 8 includes the support region 14); and a first and a second detection region 10, 12 structurally connected to the ring 8 (in particular, which extend in structural continuity of the ring 8) and likewise? coupled to pillar 4.

How will it look? evident from the present description, the element (called ?ring?) indicated with the reference number 8 can? have an open shape and, in general, different from that of a ring). For example, can have an open shape on one side (e.g., without the side opposite to the one coupled to the support frame 2); or can? be devoid of a side, for example with a ?U? shape. In the following, we will refer anyway to a ?ring? 8 without losing generality.

Each first and second sensing region 10, 12 ? mechanically coupled to the pillar 4 by means of respective anchorages 16, 18, here in particular realized as a respective pair of support arms 16 and 18. The support arms 16 extend starting from the pillar 4 towards the first detection region 10. Similarly, the support arms 18 extend from the pillar 4 towards the second detection region 12.

The dimensions of the support arms 16, 18 are chosen, as better illustrated below, in such a way that they have a lower stiffness (understood as the ability to oppose deformation when a force is applied along the direction of the Z axis). to the stiffness of the ring 8 and of the support region 14. In particular, the support arms 16, 18 have a thickness, along the Z axis, lower than the thickness of the support region 14 along the Z axis. According to a form of construction, the thickness of each support arm 16, 18 ? between 5 and 50 ?m, while the thickness of the ring 8 and of the support region 14 ? between 50 and 500 ?m.

The first and second sensing regions 10, 12 extend internally to the ring 8, and protrude toward the pillar 4 at opposite sides of the pillar 4. In other words, the first and second sensing regions 10, 12 are arranged between a respective side of the ring 8 and the pillar 4.

The first sensing region 10 includes a structural portion 10a over which extends a first sensing element 21, here in particular a piezoelectric stack which ? part of a piezoelectric transducer. The second sensing region 12 includes a respective structural portion 12a over which extends a second sensing element 23, here in particular a piezoelectric cell (?stack?) which ? part of a piezoelectric transducer.

As can be seen from the section of figure 1, the ring 8 has a thickness, along the Z axis, greater than the thickness of the first and second sensing regions 10, 12, so as to result more? rigid to deformation with respect to the first and second sensing regions 10, 12. In particular, the first and second sensing regions 10, 12 have the same thickness (along the Z axis) as the support arms 16, 18.

The support frame 2 and the pillar 4 are coupled below (in particular, glued) to a lower support 26, such as a double-sided tape, able to allow handling of the MEMS structure 50 and to form an interface for a further coupling towards a package or other substrate or body. In particular, the lower support 26 ? a tape for gluing plates or DAF (?Die Attach Tape?). As an alternative to double-sided tape 26 ? It is possible to use a substrate of semiconductor material such as silicon, or plastic or metallic material such as steel, or another support having a shape and material chosen according to need. In this case, the coupling between the support frame 2 / pillar 4 and such a substrate can be done by glue, tape, or other bonding technique. The lower support 26 ? optional, as a corresponding support could be provided and present in correspondence with the body to which the MEMS structure 50 is coupled.

A protective cap 24 extends above the support frame 2, and ? physically coupled to the support frame 2 along an annular region 27 which completely surrounds the mobile structure 6, so as to delimit a cavity internal 28; the annular region 27 ? for example, made using ?glass frit?, or other technique capable of guaranteeing fluidic insulation of the cavity? cavity 28 with respect to an environment external to it (ie ensuring that water or other fluid does not reach the internal cavity 28). The protective cap 24 extends at a distance, along the Z axis, from the mobile structure 6 and from the pillar 4. This distance is for example between 1 and 50 ?m.

During use, a pressure force F is exerted in correspondence with the lower support 26, along the axis Z (as indicated by the arrow 30 in figure 1).

This force F causes a deflection of the lower support 26 which consequently pushes the pillar 4 upwards (that is, along the axis Z, towards the protective cap 24). When the force is released, the pillar 4 returns to the starting position. The pillar 4 therefore operates similarly to a piston, moving along Z. Moving towards the cap 24, the pillar 4 causes a corresponding deflection of the support arms 16 and 18 coupled to it which, consequently, generate a corresponding deflection of the first and of the second detection region 10, 12. However, since? the first and second sensing regions 10, 12 are constrained to the ring 8 at their side opposite to that to which the arms 16, 18 are coupled, they undergo a ?strain? traction, axial type. The axial strain of the first and second sensing regions 10, 12 along the X axis? consequence of the displacement of pillar 4.

Given the greater stiffness of the ring 8 and of the support region 14 with respect to that of the support arms 16, 18 and of the detection regions 10, 12, the deformation to which the support arms 16, 18 and the support regions are subjected detection 10, 12 ? greater than that to which ? subject ring 8 (which, for the purposes of the present invention, can be defined as negligible).

The deformation, or tensile stress, to which the sensing regions 10, 12, and in particular the piezoelectric elements 21, 23, ? function of (in particular proportional to) the applied force F. The entity? of the force F can? then be detected as a function of the consequent deformation of the piezoelectric elements 21, 23 and corresponding variation of the electric signal generated by them in response to this deformation, according to the known piezoelectric principle. Figure 2 shows, in an exemplary embodiment, conductive tracks 31, 32 which connect each piezoelectric element 21, 23 to respective pads 33, 34, which can be contacted to pick up the electrical signal generated, in use by the piezoelectric elements 21, 23, in way by itself known.

The MEMS 50 structure? configured to be coupled, through the lower support 26, to a housing of a device which includes, or integrates, and uses the MEMS structure 50. Figure 3 illustrates the MEMS structure 50 of figure 1 coupled to a body 40, for example a portion of a smartwatch case, or the ?case? of a smartphone. The force F is, in this case, applied to the body 40 which, by deflecting, transfers this force to the lower support 26 and therefore to the pillar 4. The body 40 ? for example of aluminium, with greater thickness in correspondence with the support frame 2 of the MEMS structure 50 with respect to the portion in correspondence with which the pillar 4 extends. The force F is applied where the thickness of the body 40 ? minor, what? to concentrate the deflection in correspondence with the pillar 4. For example, the body 40 has a thickness equal to or greater than 500 ?m in correspondence with the support frame 2 and equal to 100 ?m in correspondence with the pillar 4.

It is clear that, by suitably modulating the thickness and stiffness of the body 40, is it possible to modulate the sensitivity? of the MEMS 50 structure.

In general, the present invention finds use in application contexts in which the forces to be detected are between 0.5 and 50 N.

With reference to figures 4-10, a method for manufacturing the MEMS structure 50 of figures 1 and 2 is now described.

With reference to Figure 4, a wafer including a silicon substrate 100 is arranged, having a front side 100a and a back side 100b, opposite each other along the Z axis. The substrate 100 has thickness, along Z , between 300 and 800 ?m. By thermal oxidation steps, a front oxide layer 102 and a back oxide layer 104 are formed on the front side 100a and back side 100b, respectively. The thermal oxidation phase can be replaced by an oxide deposition step, exclusively at the rear side 100b. It is evident that the use of silicon for the substrate 100 and of silicon oxide for the layers 102 and 104 is As an example of an embodiment, other materials typically used in the semiconductor industry may be used. How will it be? evident from the following description, the back oxide layer 104 has the function of an etch mask to protect selective portions of the substrate 100; in general, therefore, the material chosen for layer 104 ? such that the substrate 100 can be selectively attached and removed with respect to the layer 104.

Then, Fig. 5A , the back oxide layer 104 is shaped to form a mask 106. The front oxide layer is completely removed. As can be seen in figure 5B, the mask 106 has a shape and extension equal to the shape and extension designed for the ring 8, the support region 14, and the pillar 4. Figure 5A is a view along section line V-V of figure 5B .

One then proceeds with a step of growth of an epitaxial layer 108 in correspondence with the back side 10b of the substrate 100, burying the mask 106. The epitaxial layer 108 has a thickness, along Z, between 5 and 50 µm, for example equal at about 10 ?m. The epitaxial layer 108 is then subjected to a planarization process, for example by means of the CMP (Chemical Mechanical Polishing) technique, in order to planarize and/or smooth its surface. It is clear that the layer 108 can? be formed according to a technique other than epitaxy and therefore the wording of ?epitaxial layer? ? used herein as merely an example of a possible embodiment.

With reference to figure 6A (which is a view along the section line VI-VI of figure 6B), the front of the wafer is worked. To this end, a mask 110, for example of silicon oxide (or, in general, of a material which can be removed selectively with respect to the material of the substrate 100) is formed on the front side 100a of the substrate 100. Prior to forming the mask 110, the substrate 100 can? optionally be machined so as to reduce its thickness, for example to reach a final thickness along Z of between 50 and 500 ?m (that is, substantially like the support frame 2), for example equal to about 500 ?m. Figure 6B illustrates, in XY plane view, the mask 110. The mask 110 has a perimeter, in the XY plane, corresponding to the perimeter of the mask 106, and protects both the area in which it will come? formed the ring 6 and the area internally delimited by the ring 6, with the exception of the regions in which the support arms 16 and 18 will be formed (these regions are left exposed, as can be seen from figure 6B).

A step of forming an epitaxial layer 112 is performed on the front side 100a, burying the mask 110. The epitaxial layer 112 has a thickness, along Z, of between 5 and 50 ?m, for example equal to 7.5?m. It is clear that the layer 112 can? be formed according to a technique other than epitaxy and therefore the wording of ?epitaxial layer? ? used herein as merely an example of a possible embodiment.

The sensitive elements 21 and 23 are then formed. As mentioned, the sensitive elements 21 and 23 are in particular piezoelectric elements, each including a pile (?stack?) formed by an interposed piezoelectric layer (for example aluminum nitride, AlN) between a bottom electrode and an upper electrode, in a way by itself? known. With reference to Figures 7A-7D, manufacturing steps of a piezoelectric stack 115 on the epitaxial layer 112 are illustrated, illustrated limited to the portion of the wafer involved.

The manufacturing steps of the piezoelectric stack 115 therefore include a step of forming the bottom electrode 117, for example by depositing a layer of molybdenum with a thickness of about 200nm; a step for forming the piezoelectric layer 118, for example by depositing AlN on the bottom electrode layer with a thickness of about 1 µm; and a step for forming the upper electrode 119, for example by depositing a layer of molybdenum with a thickness of about 200nm on the piezoelectric layer. The 115 stack cos? format is then shaped, figure 7B, to define the desired shape of the sensitive elements 21, 23, in particular according to what has already been described with reference to Figures 1 and 2 . In Figure 7C , the shaped piezoelectric stack is then protected by a passivation layer 120. Additionally, a hard mask layer 121 (e.g., TEOS) is formed on the wafer, on top of the epitaxial layer 112 and sensitive elements 21, 23.

Furthermore, electrical contacts 122, 123 are formed between the passivation layer 120 and the rigid mask layer 121, suitable for electrically contacting respectively the upper and lower electrodes 119, 117. the conductive tracks 31, 32 are formed on the rigid mask layer 121, for example simultaneously with the formation of the electrical contacts 122, 123, electrically coupled to the electrical contacts 122, 123.

With reference to figure 8, the protective cap 24 is arranged above the epitaxial layer 112. The protective cap 24 can be manufactured by shaping a further semiconductor wafer, and performing a wafer-to-slice bonding step, so as to couple the protective cap 24 on the rigid mask layer 121 (by forming the annular region 27 described with reference to figure 1).

Then, figure 9, one proceeds with a lithography and etching step to remove selective portions of the epitaxial layers 108, 112 and of the substrate 100 not protected by the masks 106 and 110. In particular, a photoresist mask 130 is formed on the epitaxial layer 108 , so as to protect the regions intended to form the support frame 2 and leave the regions intended to form the mobile structure 6 exposed to attack (with the exception of the region intended to form the pillar 4, which is also protected by the mask 130) .

An etching is then performed from the back of the wafer to remove material (here, silicon) exposed through the photoresist mask 130. The etching is done. here of the anisotropic type, and proceeds by removing along the direction of the Z axis selective portions of the epitaxial layers 108, 112 and of the substrate 100, stopping at the masks 106 and 110, and instead proceeding with the complete removal, along the etching direction (Z axis) of the epitaxial layers 108, 112 and the substrate 100 where not protected by the masks 106 and 110. If necessary, a further etching is performed to remove any additional materials present on the epitaxial layer 112 following the previous manufacturing steps. They come like this define the support frame 2 and the mobile structure 6 (the latter, separated from the support frame 2 along the entire perimeter following the previous attachment step, with the exception of the support portion 14).

With reference to Figure 10, the masks 110 and 106 are removed by wet etching, with the exception of a portion of the mask 106 buried in the epitaxial layer 108 at the pillar 4. The presence of this portion of the mask does not imply no disadvantage in terms of operation of the MEMS structure 50.

We then proceed with the (optional) step of coupling the lower support 26 to the epitaxial layer 108, obtaining the MEMS structure 50 of figure 1.

Figure 11 illustrates in schematic form a package, or other enclosure, 200 within which the MEMS structure 50 and a controller, or ASIC, 201 are housed. The MEMS structure 50 is housed. coupled, by means of the lower support 26, to a side of the package 200 at which the force F acts. The ASIC 201 ? coupled to the same side of the package 200 through a printed circuit board 205.

The electrical connection between the MEMS structure 50 and the ASIC 201 can be implemented, internally to the package 200, by wire bonding 203, and additional conductive spheres 204 may be present to physically and electrically bond opposite sides of the package, so to make an electrical connection to the top side of the package, opposite the side at which ? the MEMS structure 50 is arranged. The inside of the package 200 can? it can also be filled with a resin 206, according to the technique known as "injection moulding".

Other forms of packaging can be envisaged, including, for example, the mounting of the MEMS structure 50 on a flexible board 210, in correspondence with the side of the MEMS structure 50 opposite to the one presenting the lower support 26, as illustrated in figure 12. In this way, the electrical connections with the conductive pads 33, 34 can be made by means of conductive balls 212 coupled to the flex board and the lower support 26 can be connected to the conductive pads. be mechanically coupled to a body (e.g., the body 40 of Figure 3) which, in use, receives the force F.

The invention and the related manufacturing process described herein have various advantages.

For example, the MEMS structure described ? impermeable to water or other liquids, therefore without having to prepare sealing elements (for example, O-rings) capable of isolating it from the external environment. Furthermore, this structure fully houseable in a package or other enclosure that is not accessible from the outside, further improving waterproofing.

The described MEMS structure has reduced overall dimensions and therefore can? also be used in small electronic devices, such as a smartwatch or a smartphone.

In addition, the force sensing principle allows for good linearity? in the answer, as well as? accuracy in determining the applied external force. Furthermore, the used force sensing principle of the allows for a good performance (?yield?).

Furthermore, the described MEMS structure can be fabricated using techniques or steps common in the fabrication of MEMS devices, therefore at low cost and exploiting the fabrication tools already? existing in many semiconductor processing plants.

Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without departing from the protective scope of the present invention, as defined in the attached claims.

Claims (21)

1. Push button device (50) comprising: - a fixed support structure (2); - a movable structure (6), laterally surrounded at a distance from said support structure (2), physically coupled to the support structure (2) through a protrusion (14), and configured to deform at least partially under the action of a external force (F); And - a cap (24), coupled by means of a fluid-tight region (27) to the support structure (2), the fluid-tight region (27) surrounding, in plan view, said movable structure (6), wherein the mobile structure (6) includes: a suspended structure (8), having a first stiffness, including a first side coupled to the support structure (2) through said protrusion (14) and at least one second and one third side extending from the first side in continuity? structural of the same; a piston element (4) configured to translate, under the action of an external force (F), along a direction parallel to said external force (F); at least a first and a second deformable element (10a, 12a) extending in continuity? structure of the suspended structure (8) starting, respectively, from the second and third sides towards the piston element (4), and having a second stiffness lower than the first stiffness; a first anchor (16), having the second stiffness, coupled between the piston element (4) and the first deformable element (10a); a second anchorage (18), having the second stiffness, coupled between the piston element (4) and the second deformable element (12a); a first sensitive element (21) coupled to the first deformable element (10a), configured to detect a deformation of the first sensitive element (21) and generate a signal indicative of said deformation of the first sensitive element (21); And a second sensitive element (23) coupled to the second deformable element (12a), configured to detect a deformation of the second sensitive element (23) and generate a respective signal indicative of said deformation of the second sensitive element (23). 2. Push-button device according to claim 1, wherein the piston element (4), the first and second deformable elements (10a, 12a), and the first and second anchor elements (16, 18) are among mutually connected in such a way that, when the external force (F) acts on the piston element (4), the first and second anchor elements (16, 18) transfer said external force (F) to the first and respectively to the second deformable element (10a, 12a), so that? the first and second deformable element (10a, 12a) are subjected to an axial deformation which can be detected by said first and second sensitive elements (21, 23). The push button device according to claim 1 or 2, wherein the first anchor (16) comprises a first pair of arms, and the second anchor (18) comprises a second pair of arms. 4. Push-button device according to any one of the preceding claims, wherein said suspended structure (8) has a thickness greater than the thickness of the first and second deformable elements (10a, 12a) and of the first and second anchoring elements (16, 18). 5. Push-button device according to any one of the preceding claims, wherein the suspended structure (8) is coupled to the support structure (2) exclusively by means of a beam extending from the first side of the support structure (2), said beam having the first stiffness. 6. Push-button device according to any one of the preceding claims, wherein the second and third sides of the suspended structure (8) laterally face opposite sides of the piston element (4). 7. Push-button device according to any one of the preceding claims, wherein the first and second sensing elements are piezoelectric transducers. 8. Push-button device according to any one of the preceding claims, further comprising a cap (24) fixed in a watertight manner to the support structure (2) and completely surrounding, in plan view, the movable structure (6). The push-button device according to any one of the preceding claims, further comprising conductive tracks extending between the first and, respectively, the second sensitive element (23) and respective electric contact pads. 10. Push-button device according to any one of the preceding claims, further comprising a bearing element (26; 40) fixed to the support structure (2) and to the piston element (4) at the portion of the piston element (4 ) configured to receive said external force (F). The pushbutton device according to claim 10, wherein said carrier (26) is a double-sided tape, such as for example a ?Die Attach Film?, DAF. 12. Push-button device according to claim 10, wherein said bearing element (40) has a first thickness in correspondence with the support structure (2) and a second thickness lower than the first thickness in correspondence with the piston element (4), in such a way as to be deformable by said external force (F) exclusively in correspondence with the piston element (4). 13. Enclosure (200) having a cavity internal and a deformable portion delimiting one side of the cavity? internal and intended to receive, in use, said external force (F), in which said cavity? internal houses: - a button device (50) according to any one of claims 1-11, coupled to said deformable portion; - a control device, or an ASIC, (201) operably coupled to the push button device; And - a filling layer (206), which completely covers said button device (50) and said control device, or an ASIC, (201). 14. Method of manufacturing a button device (50), comprising the steps of: - forming a fixed support structure (2); - forming a movable structure (6), laterally surrounded at a distance from said support structure (2) and configured to deform at least partially under the action of an external force (F); forming a protrusion (14) from the movable structure (6) towards the support structure (2), which physically couples the movable structure (6) to the support structure (2); and - coupling a cap (24), by means of a fluid-tight region (27), to the support structure (2), the fluid-tight region (27) surrounding, in plan view, said movable structure (6) , wherein the step of forming the mobile structure (6) includes the steps of: form a suspended structure (8), having a first stiffness, including a first side coupled to the support structure (2) through said protrusion (14) and at least one second and one third side extending from the first side in continuity structural of the same; forming a piston element (4) configured to translate, under the action of an external force (F), along a direction parallel to said external force (F); form at least a first and a second deformable element (10a, 12a) extending in continuity? structure of the suspended structure (8) starting, respectively, from the second and third sides towards the piston element (4), and having a second stiffness lower than the first stiffness; forming a first anchor (16), having the second stiffness, coupled between the piston element (4) and the first deformable element (10a); forming a second anchorage (18), having the second stiffness, coupled between the piston element (4) and the second deformable element (12a); forming a first sensitive element (21) on the first deformable element (10a), configured to detect a deformation of the first sensitive element (21) and generate a signal indicative of said deformation of the first sensitive element (21); And forming a second sensitive element (23) on the second deformable element (12a), configured to detect a deformation of the second sensitive element (23) and generate a respective signal indicative of said deformation of the second sensitive element (23). 15. Method according to claim 14, wherein the piston element (4), the first and second deformable elements (10a, 12a), and the first and second anchor elements (16, 18) are mutually connected in such a way that, when the external force (F) acts on the piston element (4), the first and second anchor elements (16, 18) transfer said external force (F) to the first and respectively to the second deformable element (10a, 12a), so that? the first and second deformable element (10a, 12a) are subjected to an axial deformation which can be detected by said first and second sensitive elements (21, 23). The method according to claim 14 or 15, wherein forming the first anchor (16) comprises forming a first pair of arms, and forming the second anchor (18) comprises forming a second pair of arms. 17. Method according to any one of claims 14-16, wherein said suspended structure (8) has a thickness greater than the thickness of the first and second deformable elements (10a, 12a) and of the first and second anchoring elements (16, 18). 18. A method according to any one of the preceding claims, wherein forming the second and third sides of the suspended structure (8) comprises forming the second and third sides of the suspended structure (8) laterally facing opposite sides of the piston element ( 4). The method according to any one of claims 14-18, wherein forming the first and second sensing elements includes forming respective piezoelectric transducers. 20. Method according to any one of claims 14-19, further comprising the step of fixing to the watertight support structure (2) a cap (24) which completely surrounds, in plan view, the movable structure (6). 21. A method according to any one of claims 14-20, further comprising the step of fixing a bearing element (26; 40) to the support structure (2) and to the piston element (4) at the portion of the element to piston (4) configured to receive said external force (F), said bearing element being one of: a double-sided tape, such as for example a "Die Attach Film", DAF; a support (40) having a first thickness in correspondence with the support structure (2) and a second thickness lower than the first thickness in correspondence with the piston element (4), so as to be deformable by said external force (F) exclusively at the piston element (4).